Polyomavirus nephropathy (PVN) is a form of nephritis caused by reactivation of the human polyomavirus, BKV, in renal transplant patients. BKV is ubiquitous in most human populations and establishes a lifelong, subclinical, persistent infection of the kidney. In transplant patients and other immunocompromised individuals, viral replication leads to tissue damage and renal dysfunction. The incidence of PVN has risen dramatically in recent years concomitant with rising numbers of transplants being performed and the development of more effective immunosuppressive drug regimens aimed towards preventing rejection of the transplant. Presently, there are no effective anti-viral treatments for BKV, and therefore the clinician is faced with the dilemma of reducing the dose of immunosuppressive drugs to allow the patient's immune system to battle the virus, which then raises the risk of graft rejection. The biology of BKV in human renal epithelial cells is beginning to be uncovered, and a better understanding will be critical in the design of therapeutic approaches. Using an in vitro cell culture system for the propagation of primary human renal proximal tubule epithelial cells that allows them to maintain their differentiated function, key features of the BKV life cycle can be studied. BKV replicates efficiently in these cells. Treatment of the cells with interferon-?, however, inhibits viral gene expression and, therefore, replication. The aims of this project are to utilize this in vitro system to characterize the mechanisms that regulate viral replication, including an analysis of viral chromatin and subnuclear localization of the viral genome under replicating and non- replicating conditions. In addition, the pathway that the viral particle takes from the plasma membrane to the nucleus of the cell, during which it begins to disassemble, will be examined. The long term goals of these studies are to dissect the mechanisms that regulate BKV replication in renal tubular epithelial cells and to identify steps in the viral life cycle that may be amenable to the development of new antiviral drugs.